<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            width: 400px;
            height: 400px;
            background-image: url("../../chongqing07-basic/images/act.png");
        }
        .mask {
            width: 100%;
            height: 100%;
            background-color: rgba(0, 0, 0, 0);
            transition: background-color 1s ease;
            /* display: none; */
            /* visibility: hidden; */
        }
        .box:hover .mask {
            background-color: rgba(0, 0, 0, .5);
            /* display: block; */
            /* visibility: visible; */
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="mask"></div>
    </div>
</body>
</html>